cmd/compile: use the builtin clear
To simplify the code a bit.
Change-Id: Ia72f576de59ff161ec389a4992bb635f89783540
GitHub-Last-Rev:
eaec8216be964418a085649fcca53a042f28ce1a
GitHub-Pull-Request: golang/go#73411
Reviewed-on: https://go-review.googlesource.com/c/go/+/666117
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>